Correctly skip if-null expressions.

tests/language/regress_26543_2_test.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file. 
+
+// Regression test for issue 26543
+
+class C {
+  var x, y;
+  C() : x = null ?? <int, int>{}, y = 0 {}
+}
+
+main() {
+  print(new C());
+}